How do you sculpt a texture into a 3D Model (i.e. stencil overlay)
Load your texture in Properties panel > Texture.





Switch your 3D view to Sculpt mode.
On the left (tools panel), choose the Draw brush.
Open the N panel on the right of your 3D view and click on the Tool tab. Open your texture so that the brush will use it, choose Mapping > Stencil.





The texture should be displayed on the bottom left corner of your 3D view, it might be very transparent for the moment. In the same panel as before, under Display, increase the Texture Alpha if necessary. Enable Dyntopo if your object is not high poly enough and choose the adequate Detail Size, or give your object a Multiresolution modifier.





Move your texture in the 3D view: RMB to move, Shift RMB to scale, Ctrl RMB to rotate. Paint on your object.